1. Field of the Invention
The present invention relates to an image creation program and a method of creating an image, and more particularly to image processing in a video game machine having a 3D image processing function.
2. Description of the Background Art
With recent technical advance, high-performance video game machines having built-in 3D image processing circuits, such as a geometry arithmetic circuit and a rendering circuit, have come along. In such video game machines, an image corresponding to a scene viewed from a virtual camera is created by transforming a plurality of 3D objects arranged in a 3D virtual space (the world coordinate system) into the 2D screen coordinate system which is based on the virtual camera by performing geometry calculation, and then mapping texture onto polygons of the objects having been transformed into 2D coordinates.
Conventionally, in the case of displaying a character or the like walking on the land, in particular, as described in patent document 1 (Japanese Patent No. 3068205 (FIGS. 4 and 6)) and patent document 2 (Japanese Laid-Open Patent Publication No. 2000-93654 (FIGS. 4 and 11)), a 3D game image is displayed by arranging a character object on a land object and displaying a scene of the character object and the land object viewed from a virtual camera.
As a technique for displaying a game screen using a planar polygon, there is known a conventional technique described in patent document 3 (Japanese Laid-Open Patent Publication No. 10-179932 (FIG. 4)). Making a blast pattern or the like into a 3D object makes game creation complicated. However, according to the above conventional technique, by mapping a blast pattern onto a planar polygon which look towards the viewing direction at all times, the blast pattern can be displayed in a 3D format with simple processing.
Creation of a 3D game program such as that described in patent documents 1 and 2, however, involves creating a 3D object using a plurality of polygons and creating a realistic image to be mapped onto the object. Accordingly, creating a game program takes a very long time.
In recent years, due to the aforementioned problem, even game titles for which high sales cannot be expected require a certain amount of time to develop their game programs, causing another problem that development costs cannot be recovered eventually.
There is a move to port outdated game programs which are developed for low-function game machines having no built-in 3D image processing circuits or for outdated game machines. However, the game programs of such game machines include only 2D image data, such as characters and land, and do not include 3D objects composed of polygons and realistic texture to be mapped onto the objects. Therefore, it is not easy to port the game programs of such game machines to game machines having built-in 3D image processing circuits.
To overcome the above problems, with reference to patent document 3, an image of a character may be mapped onto a non-dimensional, planar polygon, whereby porting of game programs of outdated game titles may be realized or the development time for game programs may be reduced.
As is clear from patent documents 1 and 3, technically, it is common practice in conventional 3D game programs to arrange character objects such as people and blasts to be displayed on the land, on a land object and arrange birds, in-air blasts, or the like at a location away from the land object.
In the case, for example, of porting an outdated game program using a 2D image included therein, however, the following problem occurs.
For example, in recent 3D games, in a virtual space including a virtual camera 1601, a character object 1602, an another object 1603, and a land object 1604, as shown in FIG. 16, when the character object 1602 which is holding a sword longer than half length thereof brings down the sword forward, as shown in FIG. 17, the sword is brought down in a diagonal arc and thus the resulting image to be displayed is not such that the long sword is sinking into the ground.
Now, the case where as in patent document 3, planar polygons are used in place of the 3D objects in FIG. 16 will be discussed. In this case, as shown in FIGS. 18A and 18B, planar polygons 1801 and 1802 are prepared onto which texture viewed from a virtual camera are mapped on an object-by-object basis. Then, the polygons 1801 and 1802 are superimposed and displayed such as that shown in FIG. 18C. FIG. 19 is a diagram showing the planar polygons 1801 and 1802 arranged in a manner shown in FIG. 18C as viewed from a direction perpendicular to the virtual camera. Similarly, in the case of expressing an action of the character bringing the long sword down, as shown in FIG. 20A, a texture of the character performing such an action is mapped onto a planar polygon 2001 (which may be the same polygon as the planar polygon 1801) and then the planar polygons 2001 and 1802 are superimposed and displayed. Here, if the planar polygon 2001 is arranged such that the character is standing in the same position as that of FIG. 18C, the planar polygon 2001 turns out to sink into the ground as shown in FIG. 20C. Accordingly, the resulting image is displayed such that the sword brought down in a diagonal arc by the character sank into the ground.
In a 2D virtual space, even if using an image such that the edge of a sword is located below the feet of a character, no particular problems arise. However, as in the above case, when in a 3D virtual space, an image such that the edge of a sword is located below the feet of a character is mapped, as texture, onto a polygon arranged so as to stand on a land object, a problem that a part of the sword is not displayed arises. The problem is more serious when an image of a character throwing something longer than a sword (e.g., a lasso) is mapped onto a polygon as texture.